Re: Simple but Fundamental Antenna Question

From: andy (news4_at_earthsong.free-online.co.uk)
Date: 07/25/04 

Date: Sun, 25 Jul 2004 14:34:45 +0100

On Sun, 25 Jul 2004 02:05:58 +0100, John Jardine wrote:

>
> John Popelish <jpopelish@rica.net> wrote in message
> news:855a1a14.0407240659.79813146@posting.google.com...
>> j2israel@hotmail.com (JLD) wrote in message
> news:<aecbba1f.0407221413.1520a904@posting.google.com>...
>> > Hello all.
>> >
>> > I have never studied RF communications. With that said I have a
>> > question about how antennas work, and I am talking about simple
>> > antennas (simple straight wires) like on a car.
>> > How can an AC current exist in a wire that is connected to some
>> > circuitry at one end but is unconnected, open, floating at the other
>> > end? How can the current alternate back and forth in an open wire?
>> (snip)
>>
>> You have run up against one of the limitations of viewing electric
>> circuits as consisting of lumped components (resistors, capacitors,
>> inductors, etc.), only. Since electric energy gets from place to
>> place as a wave, traveling at the speed of light, an understanding of
>> things like transmission lines, antennas and free traveling waves have
>> to take this property of space, matter and time into effect if it is
>> going to make any sense at all. A whip antenna may be visualized from
>> the energy's point of view (and as a mental transition from lumped
>> circuits) as a long chain of little inductors with small capacitors
>> connected, all along the line between inductors into nearby space
>> (essentially one plate capacitors). If the antenna were surrounded by
>> a tube of conductor, it would be easier to picture where the far end
>> of those capacitors connected, but that would be a tansmission line,
>> rather than anatenna.
>> The property that makes a whip an antenna instead of a transmission
>> line is that the magnetic fields of those tiny inductances and the
>> electric fields of those tiny capacitances do not terminate into a
>> conductive container, but into waves of energy that propagate away
>> from the antenna, always leaving the space next to the antenna
>> available to soak up magnetic and electric fields from the next part
>> of the cycle. Those waves consist of globs of electric and magnetic
>> fields that build each other in the direction they are moving and
>> extinguish each other where they have been.
>
> What I've never really been able to rationalise, is that we know with
> certainty that real energy is being radiated from the aerial yet we're told
> that E=MC^2 no longer applies, as the EM 'energy' travels at light speed
> therefore can't have mass. Photon thingies are then invoked. I'd feel easier
> if I knew the power supply was losing mass all the time the transmitter was
> switched on. (like Newtonian mechanics and powering a rocket by throwing
> mass out of the back).
> regards
> john

i think it is, isn't it - not by losing particles, but by losing the
E=MC^2 mass-energy of their chemical energy (if it's a battery)

also, the thing photons don't have is rest-mass, but they do have a
mass-energy related to the wavelength of light they are carrying. i think,
from my somewhat flaky memory of this stuff. 

-- 
http://www.niftybits.ukfsn.org/
remove 'n-u-l-l' to email me. html mail or attachments will go in the spam
bin unless notified with [html] or [attachment] in the subject line.

Relevant Pages

  • Re: Simple but Fundamental Antenna Question
    ... > circuits as consisting of lumped components (resistors, capacitors, ... antennas and free traveling waves have ... A whip antenna may be visualized from ... > available to soak up magnetic and electric fields from the next part ...
    (sci.electronics.basics)
  • Re: Simple but Fundamental Antenna Question
    ... circuits as consisting of lumped components (resistors, capacitors, ... antennas and free traveling waves have ... A whip antenna may be visualized from ... available to soak up magnetic and electric fields from the next part ...
    (sci.electronics.basics)
  • Re: Hey BMJ....
    ... I was thinking about capacitors used like a T or gamma match. ... One reason I consider using a tuner is that I could set up my antenna, tune it properly with my antenna analyzer one day, set it up again to the way I had it before another day and the SWR's gone to pot because a neighbour put something out on the balcony. ...
    (sci.research.careers)
  • Re: My capacity to react
    ... but in reality all capacitors & inductors have some ... That resistance will offset the phase shift slightly. ... It is important to note that the losses in an inductor are not limited ...
    (alt.guitar.beginner)
  • Re: Hey BMJ....
    ... Then go to the variable capacitors and adjust one or the other to see if you can get the reflected power down some more. ... One reason I consider using a tuner is that I could set up my antenna, tune it properly with my antenna analyzer one day, set it up again to the way I had it before another day and the SWR's gone to pot because a neighbour put something out on the balcony. ... It should not affect tuning that much unless you get something very close to the antenna. ...
    (sci.research.careers)